ipv4: Create and use fib_compute_spec_dst() helper.

The specific destination is the host we direct unicast replies to.
Usually this is the original packet source address, but if we are
responding to a multicast or broadcast packet we have to use something
different.

Specifically we must use the source address we would use if we were to
send a packet to the unicast source of the original packet.

The routing cache precomputes this value, but we want to remove that
precomputation because it creates a hard dependency on the expensive
rpfilter source address validation which we'd like to make cheaper.

There are only three places where this matters:

1) ICMP replies.

2) pktinfo CMSG

3) IP options

Now there will be no real users of rt->rt_spec_dst and we can simply
remove it altogether.

Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/ip_fib.h

@@ -230,6 +230,7 @@ extern struct fib_table *fib_get_table(struct net *net, u32 id);
 /* Exported by fib_frontend.c */
 extern const struct nla_policy rtm_ipv4_policy[];
 extern void		ip_fib_init(void);
+extern __be32 fib_compute_spec_dst(struct sk_buff *skb);
 extern int fib_validate_source(struct sk_buff *skb, __be32 src, __be32 dst,
 			       u8 tos, int oif, struct net_device *dev,
 			       __be32 *spec_dst, u32 *itag);